issue-1363: Allow null in "selectedKey" to avoid typing problems, doc… (#2676)
* issue-1363: Allow null in "selectedKey" to avoid typing problems, document using "null" to clear key selection
* remove workaround example
* remove workaround story
* Update packages/@react-spectrum/picker/docs/Picker.mdx
Co-authored-by: Robert Snow <rsnow@adobe.com>
Co-authored-by: Danni <darobins@adobe.com>
Co-authored-by: Danni <drobinson@livefyre.com>